In a data storage system, data can be stored on a mass-storage device or in a semiconductor memory. Mass-storage devices, such as disks, offer considerable capacity at low cost per unit of data stored, but long access times. In contrast, semiconductor memory provides much shorter access times at higher costs per unit of data stored.
Many database systems permit a user to write specialized database applications. These database applications often perform queries that require retrieval of records from one or more mass-storage devices. Each retrieval of a record requires accessing the mass-storage device to copy data to a semiconductor memory. In processing such queries, it is desirable to pre-fetch records and store them in the semiconductor memory. This reduces latency because the database application can then process records already in memory while another process pre-fetches additional records.
Pre-fetching records is effective at reducing latency only to the extent that the pre-fetched records are actually sought by the database application. The success of a pre-fetch policy thus depends on the extent to which one can correctly anticipate the needs of a database application.